Because of the unique stresses of the war in Vietnam and the focus
of home front frustration on the returning veterans, may of those who
served in Southeast Asia have had difficulty adjusting. Many
groups have formed to support and address the issues of the
Vietnam-era veteran. Here are links to just a few.

Many feared that the use of chemical defoliants in Southeast Asia
would open the United States to changes of chemical warfare.
That turned out to be the least of our worries.
Since the end of the war, Agent Orange has been proven to
contain a component (dioxin) which is a known carcinogen.
Some claim that this fact was known when the decision to use Agent
Orange was made.
Agent Orange and its adverse medical impact has become a veteran
issue as well as an international health concern.

Much of the controversy over the Vietnam War centered on the Selective Service System -- the draft.
The transition to an all volunteer force in the 1970's was an attempt to remove selective service as a social issue.
As America again faces the prospect of an extended international conflict, the reinstatement of the draft has become a visible issue.

Failure to achieve US policy objectives in Vietnam through the use of military power led to analysis and speculation about why the failure occurred and under what circumstances the United States could effectively employ military power in the future. In remarks to the Washington Press Club on 28 November 1984, then-Secretary of Defense Caspar Weinberger proposed the following six criteria to determine the conditions under which the use of military force was warranted:

The United States should not commit forces to combat overseas unless the particular engagement or occasion is deemed vital to our national interest or that of our allies. . . .

If we decide it is necessary to put combat troops into a given situation, we should do so wholeheartedly and with the clear intention of winning.
. . .

If we do decide to commit forces to combat overseas, we should have clearly defined political and military objectives. . . .

The relationship between our objectives and the forces we have committed--their size, composition, and disposition--must be continually reassessed and adjusted if necessary. . . .

Before the United States commits combat forces abroad, there must be some reasonable assurance we will have the support of the American people and their elected representatives in Congress. . .

The commitment of US forces to combat should be a last resort.[7]

Mr. Weinberger used these criteria--the so-called "Weinberger Doctrine"--to address the primary reasons that knowledgeable people agreed had led to the US failure in Vietnam.
He continued to assert the idea, pervasive during the Cold War, that military force could only be used in pursuit of vital national interests.
No doubt this idea seemed relevant, since any use of military power in those times carried the threat of superpower confrontation and escalation to war in which the very survival of the nation could be at risk.
Additionally, Weinberger proposed in his sixth criterion that the use of military force be a policy option of last resort.
This limited the available means with which the United States could initially pursue policy.
However, in the Cold War context, these six criteria or tests were meant to prevent the United States from being caught up in a "gradualist incremental approach" to the use of military power--an approach that more often than not led to insufficient use of force.[8]

This document is sometimes referred to as the Powell-Weinberger Doctrine. General Powell was Secretary of Defense Weinberger's miitary assistant and drafted this doctrine.

The decision of President Johnson to commit America to a major war
without the consent of Congress was another major controversy
resulting from the Vietnam War. Congress corrected the
situation with the War Powers Act * in 1973, but this law itself has
been the focus of debate in recent years over U.S. operations around the world.